The present invention relates to a control apparatus for a spot welding robot particularly capable of controlling opening/closing operations of tips of a direct spot welding gun without utilizing a hydraulic or pneumatic controlling device.
In a conventional art of an automobile manufacturing line, for example, spot welding robots R have been utilized. Such a spot welding robot utilizes a spot welding gun operated by hydraulic or pneumatic means as shown in FIG. 7, which is a schematic diagram of a conventional spot welding robot controlling system. The system of FIG. 7 comprises a hydraulic or pneumatic tip elevating unit 101, a tip elevating member 102, a pneumatic or hydraulic control unit 103, called fluid control unit hereinafter and a robot control unit 104. The hydraulic control unit 103 has a construction basically different from a robot control unit 104 which is mainly composed of electric elements or parts. Therefore, the hydraulic control unit 103 cannot be integrally combined with the robot control unit 104, and hence, these control units 103 and 104 are independently installed. In an operation of welding a workpiece W, the robot control unit 104 controls a control element of the hydraulic control unit 103 thereby indirectly controlling the operation of a spot welding gun G having a tip G1 which opens and closes in both the directions shown by an arrow onto tip 621, thus making the control system more complex, resulting in increased cost of manufacture and maintenance of the entire apparatus.
Furthermore, the hydraulic control unit 103 of the conventional apparatus includes piping means, cylinder means and valve means, each of which has a certain capacity or volume, resulting in that time delay in operation may be likely caused. Accordingly, it is difficult to synchronously control the hydraulic control unit with the robot control unit mainly composed of electric elements having less time delay in operation. For this reason, in the prior art, as shown in FIG. 8, because it is difficult to perform synchronous control it is not possible to perform opening, closing and welding operation of the spot welding gun G while the robot is moving. Therefore, the robot has to be first moved without operating the gun at this time and moved to the position for welding. Then, only the gun has to be moved and the series of operations of closing.fwdarw.welding.fwdarw.opening are then performed. In particular, the following steps are performed:
Step 1: The robot is at standby at the original point. PA0 Step 2: Axis 1 and axis 2 are started and the robot is moved to the escape point close to the welding point, then to the welding point. PA0 Step 3: The gun is closed after the robot has arrived at the welding point, and the operation commences. PA0 Step 4: The gun is closed, and the welding starts after the work is held at a required pressure. PA0 Step 5: The welding finishes. PA0 Step 6: The gun opening work starts, and after the gun has opened to a certain extent, the robot operation for moving to the escape point close to the welding point and to the origin starts. PA0 Step 7: The robot returns to the origin, and axis 1 and axis 2 stop. PA0 a spot welding gun having a spot welding tip; PA0 an electric drive means for driving the tip; and PA0 a control means for controlling the electric drive servo mechanism, PA0 wherein the operation of the spot welding gun is synchronously controlled with that of the robot as one axis means of the robot.
Thus this requires much time for the spot welding, resulting in a significant problem in an automobile industry in which an improved production efficiency on the production line is increasingly required.
In order to obviate such defects of a spot welding gun G utilizing the fluid control unit 103 of the prior art, there is provided a spot welding gun utilizing an induction motor as disclosed in the Japanese Patent Publication (KOKOKU) No. 3-50631 (50631/1991).
However, a spot welding gun utilizing the induction motor includes a servo mechanism, which differs from that of the robot control unit, and accordingly, in order to synchronously operate such a spot welding gun utilizing the induction motor with the robot control unit, another independent control unit is required between this spot welding gun and the robot control unit, thus increasing the cost and making the apparatus itself complicated. Thus, synchronous operation cannot be easily achieved and the entire welding time cannot be remarkably shortened. Therefore, it is difficult to directly or easily apply conventional spot welding guns to spot welding robots.